MSC4108 relies on ETag to determine if something has changed on the
rendez-vous channel.
Strong and correct ETag comparison works if the response body is
bit-for-bit identical, which isn't the case if a proxy in the middle
compresses the response on the fly.

This adds a `no-transform` directive to the `Cache-Control` header,
which tells proxies not to transform the response body.

Additionally, some proxies (nginx) will switch to `Transfer-Encoding:
chunked` if it doesn't know the Content-Length of the response, and
'weakening' the ETag if that's the case. I've added `Content-Length`
headers to all responses, to hopefully solve that.

This basically fixes QR-code login when nginx or cloudflare is involved,
with gzip/zstd/deflate compression enabled.

Keep up-to-date with pyo3 releases. This bump enables Python 3.13
support and resolves deprecations.

Links for quick reference:
https://github.com/PyO3/pyo3/releases
https://github.com/davidhewitt/pythonize/releases
https://github.com/vorner/pyo3-log

This is a workaround for some proxy setup, where the ETag header gets
stripped from the response headers unless there is a Content-Type header
set.

In particular, we saw this bug when putting Cloudflare in front of
Synapse.
I'm pretty sure this is a Cloudflare bug, as this behaviour isn't
documented anywhere, and doesn't make sense whatsoever.

---------

Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

This version change requires a migration to a new API. See
https://pyo3.rs/v0.21.2/migration#from-020-to-021

This will fix the annoying warnings added when using the recent rust
nightly:

> warning: non-local `impl` definition, they should be avoided as they
go against expectation

Co-authored-by: Hugh Nimmo-Smith <hughns@element.io>
Co-authored-by: Hugh Nimmo-Smith <hughns@users.noreply.github.com>
Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>